In general, the refractive index of an optical material, such as a lens material or an organic resin, gradually increases with a decrease in wavelength. As indicators showing this wavelength dispersion of the refractive index, for example, Abbe number (νd) and secondary dispersion characteristics (θg,F) are known. The Abbe number and the θg,F value are specific to each optical material, but many materials have them within a certain range. The secondary dispersion characteristics and Abbe numbers of known optical materials (lens material and organic resins) are shown in FIG. 1. In FIG. 1, ▴ denotes glass, and □ denotes resins.
The Abbe number (νd) and the secondary dispersion characteristics (θg,F) can be represented by the following equations:Abbe number[νd]=(nd−1)/(nF−nC)
Secondary dispersion characteristics [θg,F]=(ng−nF)/(nF−nC)
(nd denotes the refractive index at a wavelength of 587.6 nm; nF denotes the refractive index at a wavelength of 486.1 nm; nC denotes the refractive index at a wavelength of 656.3 nm; and ng denotes the refractive index at a wavelength of 435.8 nm.)
However, optical materials (e.g., lens materials and organic resins) having high θg,F characteristics deviating from the above-mentioned certain range have been synthesized by designing in detail structures (kinds of materials and molecular structures) of the optical materials. For example, polyvinyl carbazole (“A” in FIG. 1), which is an organic resin, has the highest θg,F characteristics in the organic resin materials.
In general, in a refraction optical system, chromatic aberration is decreased by combining lens materials having different dispersion characteristics. For example, in an objective lens of a telescope, the chromatic aberration appearing on the axis is corrected by combining a positive lens element of a low-dispersion lens material and a negative lens element of a high-dispersion lens material. Therefore, when the structure of a lens or the number of lens elements is restricted or when the lens material to be used is limited, it may be very difficult to sufficiently correct chromatic aberration. As one method of solving these problems, optical elements of a glass material having anomalous dispersion characteristics have been designed.
In production of an optical element having, for example, an aspheric surface shape that is excellent in chromatic aberration correction function, for example, formation of an organic resin on spherical surface glass is excellent in mass productivity, moldability, the degree of freedom in shape, and lightness and is therefore advantageous, compared to use of a lens material. However, known organic resins have optical characteristics belonging to the certain range as shown in FIG. 1, and organic resins having discriminating dispersion characteristics are very rare.
Under such background, PTL 1 describes an optical resin composition composed of N-acryloylcarbazole, a multifunctional polyester acrylate, dimethylol tricyclodecane diacrylate, and a polymerization initiator at a predetermined ratio. It is reported that the optical resin composition has good workability and becomes, in its hardened state, a material having sufficient anomalous dispersion characteristics and durability.
On the other hand, the present inventors have focused on the fact that in order to obtain an optical element having a chromatic aberration correction function that is higher than ever, it is significantly effective for optical design that the secondary dispersion characteristics represented by the θg,F value as the material characteristics of an optical element are larger (high θg,F characteristics) so as to deviate from those of general-purpose materials. Specifically, characteristics shown as the range B (νd<25 and θg,F>0.73) in FIG. 1, where the relationship between νd and θg,F deviates from the plots of general-purpose materials, lens materials and organic resins, are significantly effective.
However, currently, there is no material having characteristics (high θg,F) shown as the range B in FIG. 1 and practical utility (low coloring and stability). Note that materials recently provided by Patent Literature 1 all have θg,F values of not larger than 0.70.